The latest version of Selenium IDE can be downloaded from the official Selenium website at SeleniumHQ.org.
Selenium IDE is an Addon for Firefox and is packaged in an XPI file. This XPI file is an universal installer and runs on all platforms that Firefox runs on. The same file would install on all common operating systems like Windows, Mac OS X and many flavours of Linux.
Once you have the XPI file, you can install it by opening it in Firefox using the File – Open File … menu item or simply draging and droping the XPI file on to any open tab in Firefox. You will get a confirmation dialog to confirm that you want to install the Selenium IDE. You will need to restart Firefox to complete the installation of Selenium IDE.
Once installed, you should see the Selenium IDE button on the Firefox toolbar. You can also start Selenium IDE from the Tools menu in Firefox by selecting the Selenium IDE menu item.
What if I cannot find Selenium IDE on the official download page?
Selenium IDE is going to be out of official support soon and it may happen that Selenium IDE can no longer be downloaded from the official download page. As the current maintainer of Selenium IDE, I plan to continue working on Selenium IDE and release new versions in the future. Check my blog and this page in case you need to download the latest version of Selenium IDE. I will keep this page updated when things change.